This invention relates to a screw press dehydrator to be used for removal of water from sewage sludge or various industrial waste sludges or for solid-liquid separation of sludgy materials during the course of manufacture of various products. More particularly, this invention relates to a screw press dehydrator provided with a cake cutter which is capable of causing the wet cake, which is being dehydrated by the pressure applied thereto with a screw within a screw barrel in the axial direction thereof, to be alternately expelled expansively toward the periphery of the screw barrel and repelled contractively inwardly repeatedly in concert with the aforementioned pressing in the axial direction thereby effecting required dehydration of the wet cake with efficiency improved over the heretofore attainable level.
A screw press dehydrator provided with a cake cutter has heretofore been proposed in Japanese Patent Publication No. 156/1980, wherein a multiplicity of stationary plates and sliding plates are alternately disposed round a screw barrel at a position closer toward the discharge outlet side thereof. The stationary plates each contain a circular hole and the sliding plates each containing an oblong hole terminating in semicircles of a radius equal to the radius of the aforementioned circular hole in the stationary plates. A screw is laid through the alternating circular and oblong holes respectively of the stationary and sliding plates, so that the sliding plates may be reciprocated, as simultaneously slid against the adjacent stationary plates, in the direction of the major axes of their oblong holes between the positions at which the opposite semicircles at the ends of the oblong holes in the sliding plates coincide with the corresponding halves of the circles of the circular holes in the stationary plates.
The aforementioned screw press dehydrator is provided with a cake cutter, wherein the rotation of the screw causes the wet cake to be pressed inside the screw barrel in the axial direction thereof and the reciprocating sliding motion of the sliding plates causes the wet cake inside the oblong holes therein to be alternately expelled expansively toward the periphery of the screw barrel and repelled contractively inwardly repeatedly. As a result, the removal of water from the wet cake is effected more efficiently than when the wet cake is dehydrated solely by virtue of the pressure applied by the screw. The water lodged in the deep portion of the wet cake migrates sparingly to the surface portion and, therefore, is not readily separated from the sludge. When the wet cake is repeatedly expelled expansively and repelled contractively alternately as described above, the oscillation thus imparted to the wet cake accelerates the migration of the water lodged in the deep portion of the wet cake to a point where the removal of water from the wet cake will be effected with enhanced efficiency.
In the aforementioned screw press dehydrator provided with the cake cutter, the expansive expulsion and the contractive repulsion of the wet cake alternately carried out vertically or horizontally in an intermittent manner by the reciprocating sliding motion of the sliding plates, causes a disadvantage in that the wet cake is violently pulsated and the spaces intervening between the stationary plates and the slding plates, which are primarily intended for escape of the water component of the wet cake, suffer from heavy leakage of the solid component of the wet cake. Moreover, the impacts of the violent pulsation of the wet cake detracts from the service life of the dehydrator. Further, since the alternating expansive expulsion and contractive repulsion of the wet cake occur only in one vertical or horizontal direction, uniform migration of the water component from the deep portion of the wet cake to the entire surface of the mass of wet cake cannot be obtained and, therefore, the efficiency of the dehydrator can be improved.
Another screw press dehydrator has been proposed, in Japanese Patent Publication No. 5819/1975, wherein a multiplicity of stationary plates and sliding plates are alternately disposed around a screw barrel at a position close to the discharge outlet side thereof. The stationary plates and the sliding plates each contain a circular hole of a fixed diameter, a screw laid through the circular holes of the alternating stationary and wherein sliding plates, and the sliding plates are oscillated around the centers of their circular holes which are kept in coincidence with the circular holes of the stationary plates.
In the screw press dehydrator just described, the sliding plates are oscillated solely for the purpose of preventing the spaces between the stationary plates and the sliding plates, i.e. the openings intended for escape of the water component of the wet cake, from being clogged with solid particles, the sliding plates are not intended to alternately expel the wet cake expansively and repel it contractively. Thus, this screw press dehydrator is inferior in terms of efficiency of water removal to the screw press dehydrator which is provided with the aforementioned cake cutter.